1. Field of the Invention
The present invention relates to an apparatus for stabilizing lead frame elements against a heater block or a semiconductor die active surface during wire bonding operations wherein the lead fingers of the lead frame are electrically connected to the bonding pads of a semiconductor die by bond wires.
2. State of the Art
Typical wire bonded semiconductor devices, such as plastic encapsulated integrated circuit devices, utilize a component called a lead frame for physical support and electrical connection of a semiconductor die to external circuitry. Typically, the lead frame is formed in a strip, with others of like configuration, from a single continuous ribbon of stamped metal. The lead frames are generally stamped or etched from copper alloy, so-called Alloy 42, red brass, stainless steel, or the like. The lead frame structure includes an outer supporting frame, a central semiconductor die supporting pad (also termed a tab, island or paddle) and a plurality of inwardly-extending lead fingers terminating short of the perimeter of the die attach location and each having a terminal bonding portion at the inner lead end. The outer supporting frame is ultimately removed subsequent to encapsulation of the wire-bonded die and leads, and forms no part of the finished device.
In the assembly of semiconductor devices utilizing such lead frames, a semiconductor die is secured to the central supporting pad (such as by a solder or epoxy die-attach, although a double-sided adhesive tape-type attach has also been suggested in the art) and then the entire lead frame, with the semiconductor die thereon, is placed into a wire bonding apparatus including a clamp assembly for holding the lead frame and die assembly, and clamping the lead fingers for bonding.
U.S. Pat. No. 4,862,245 issued Aug. 29, 1989 to Pashby et al. illustrates a so-called "leads over chip" arrangement ("LOC") on the semiconductor die (see FIG. 5). A plurality of lead fingers 106 extend over the active surface of a semiconductor die 100 toward a line of bond pads 104 wherein bond wires 102 make the electrical connection between the lead fingers 106 and the bond pads 104. An alpha barrier 108 such as a polyimide (for example, Kapton.TM.) tape is adhered between the semiconductor die 100 and the lead fingers 106. This configuration, which eliminates the use of the previously-referenced central die attach pad, may assist in limiting the ingress of corrosive environment contaminants, achieve a larger portion of the lead finger path length encapsulated in the packaging material, and reduce electrical resistance caused by the bond wires 102 (i.e. the longer the bond wire, the higher the resistance) and potential wire sweep problems aggravated by long wire loops.
In a standard wire bonding process, the bond wires are attached, one at a time, from each bond pad on the semiconductor die and to a corresponding lead finger. The bond wires are generally attached through one of three industry-standard wire bonding techniques: ultrasonic bonding--using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding--using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding--using a combination of pressure, elevated temperature, and ultrasonic vibration bursts.
To form a good bond during the wire bonding processing, it is preferable to perform the bonding at an elevated and somewhat stable temperature. Therefore, as noted above, the lead frame assembly including the attached semiconductor die is generally placed on a heater block. The semiconductor die is then clamped (via the lead frame) to the heater block by the clamping assembly. With a conventional lead frame, the lead fingers are clamped directly against the underlying heater block. In an LOC structure, the lead fingers are biased between the clamp and the active surface of the semiconductor die heater block.
In an LOC structure, the Kapton.TM. tape comprising the alpha barrier or dielectric between the semiconductor and the lead fingers becomes soft at the elevated temperature. The softening of the tape allows the lead fingers and/or semiconductor die to move in response to ultrasonic energy or pressure (force) exerted by the wire bonding head (capillary). As a result, the mechanical integrity of the wire bond to the lead fingers is diminished. Furthermore, a "bouncing" motion is imparted to the lead fingers by the wire bonding head movement, which motion may be exacerbated by the heat softened tape. This bouncing motion can also result in poor wire bonds which subsequently fail.
Thus, die fabricators are somewhat compelled to select the die attach compound (or other means) and alpha barrier tape based on the thermal stability of the materials rather than on the basis of the most effective material for a given application.
Therefore, it would be advantageous to develop an apparatus for stabilizing the semiconductor die and the lead fingers during the wire bonding process to minimize or eliminate the bouncing motion and/or the movement of the semiconductor die or the lead fingers and thus compensate for the inherent deficiencies of the clamping and wire bonding process.
There have been several previous attempts to overcome the foregoing problems. For example, for bonding LOC structures, rigid clamping plates having bond site windows therein have been reconfigured so that the bond site window is reduced in size and the downwardly-extending lip or periphery contacts the lead fingers extending over the die and clamps the lead fingers directly thereto. However, the rigid clamp has been found to be too rigid and unyielding for use with an LOC configuration and may possibly damage the die. Moreover, it has been found that the use of a rigid clamp does not necessarily ensure that each lead finger will in fact be immobilized, due to dimensional variations in lead thickness, tape thickness and die surface topography.
It has also been proposed to employ resilient elastomeric elements or other compliant material at the bottom of the lip of a rigid clamp to ensure more consistent contact with lead fingers in clamping against a die or heater block. However, it has been ascertained that the compliant materials soon lose their elasticity, conform to the underlying lead finger pattern and thus become ineffective.
U.S. Pat. No. 4,821,945 discloses a bond head assembly including an integral, vertically-movable and selectively rotatable clamp adjacent and partially surrounding the capillary to clamp individual inner lead finger ends when each wire is bonded to a lead finger. The disclosure is directed, in part, to clamping an LOC structure for wire bonding.
U.S. Pat. No. 5,035,034 discloses a modified clamp using a changeable insert on the underside thereof, the insert having a plurality of resilient fingers to clamp the individual leads of a lead frame. This structure is employed in lieu of a conventional clamp structure using a lip or extended bond site periphery to clamp the lead fingers. It appears that clamping an LOC structure for wire bonding is not specifically contemplated.
U.S. Pat. No. 5,193,733 discloses several different clamp structures, including a primary clamp structure using an extended lip to clamp inner lead finger ends of a conventional (non-LOC) lead frame in combination with a leaf-spring type auxiliary clamp secured to the primary clamp to contact the die attach pad. Also disclosed is an arrangement wherein leaf-spring type or coil-spring biased clamping structures are secured to a primary clamp without the conventional lip or extended periphery, the spring-biased clamps serving to press the lead fingers, either individually or in groups, against the heater block. Again, no consideration was apparently given to clamping an LOC structure for wire bonding.